Internal combustion engines, such as four-stroke internal combustion engines, comprise one or more cylinders and a piston arranged in each cylinder. The pistons are connected to a crankshaft of the engine and are arranged to reciprocate within the cylinders upon rotation of the crankshaft. The engine usually further comprises one or more inlet valves and outlet valves as well as one or more fuel supply arrangements. The one or more inlet valves and outlet valves are controlled by a respective valve control arrangement usually comprising one or more camshafts rotatably connected to a crankshaft of the engine, via a belt, chain, gears, or similar. A four-stroke internal combustion engine completes four separate strokes while turning a crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The uppermost position of the piston in the cylinder is usually referred to as the top dead center TDC, and the lowermost position of the piston in the cylinder is usually referred to as the bottom dead center BDC.
The strokes are completed in the following order, inlet stroke, compression stroke, expansion stroke and exhaust stroke. During operation of a conventional four-stroke internal combustion engine, the inlet valve control arrangement controls inlet valves of a cylinder to an open state during the inlet stroke of a piston within the cylinder, to allow air, or a mixture of air and fuel, to enter the cylinder. During the compression stroke, all valves should be closed to allow compression of the air, or the mixture of the air and fuel, in the cylinder. If the engine is in a power producing state, fuel in the cylinder is ignited, usually towards the end of the compression stroke, for example by a spark plug or by compression heat in the cylinder. The combustion of fuel within the cylinder significantly increases pressure and temperature in the cylinder. The combustion of the fuel usually continues into a significant portion of the subsequent expansion stroke. The increased pressure and temperature in the cylinder obtained by the combustion is partially converted into mechanical work supplied to the crank shaft in the expansion stroke. Obviously, all valves should remain closed during the expansion stroke to allow the increased pressure and temperature to be converted into mechanical work. The expansion stroke is also usually referred to as the combustion stroke, since usually, the majority of the combustion takes place during the expansion stroke. In the subsequent exhaust stroke, the exhaust valve control arrangement controls exhaust valves of the cylinder to an open state to allow exhaust gases to be expelled out of the cylinder into an exhaust system.
During normal engine braking, occurring for example when a driver of a vehicle releases an accelerator pedal, the engine will continue to operate in the above described strokes, with the exception that, normally, no fuel is supplied to the engine during engine braking, and consequently, no combustion will take place during the end of the compression stroke or during the expansion stroke. In this condition, the engine will provide some braking torque due to internal friction and due to the pumping of air from the inlet to the exhaust, in the respective inlet stroke and exhaust stroke. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. However, almost all of the energy stored in the compressed gases is returned to the crank shaft on the subsequent expansion stroke. Thereby, during normal engine braking, the compression stroke together with the subsequent expansion stroke, will not contribute to a significant braking torque of the engine.
A compression release engine brake, frequently called a Jake brake or Jacobs brake, is an engine braking mechanism used in some engines. When activated, it opens exhaust valves in the cylinders after the compression stroke, releasing the compressed air trapped in the cylinders to the exhaust system. Thereby, the energy stored in the compressed gases during the compression stroke will not be returned to the crank shaft on the subsequent expansion stroke, which increases the braking torque of the engine.
In some arrangements, the exhaust valves may be deactivated, so that they remain closed during the exhaust stroke. Usually, this is achieved using a so called lost motion arrangement, which when actuated is arranged to not transfer motion caused by an exhaust cam lobe to the exhaust valve. The air in the cylinders will thereby be compressed also during the exhaust stroke. By using a mechanism opening exhaust valves near the end of the exhaust stroke, the compressed air trapped in the cylinders is released to the exhaust system. Such arrangement almost doubles the braking torque since compression and release events are performed in the compression stroke as well as in the exhaust stroke.
The document WO2015084243 A2 relates to a four-stroke combustion engine, which instead of deactivating the exhaust valves in the exhaust stroke, performs a phase-shift of a camshaft arranged to control opening of exhaust valves relative the crank shaft to a state, where the at least one exhaust valve is controlled in such a way, that it is opened during the expansion stroke of the engine and closed during the exhaust stroke of the engine, to achieve engine braking through compression in the cylinders during the exhaust stroke. This solution provides several advantages over traditional engine braking mechanisms used, such as controllability of the size of the braking torque.
Due to environmental concerns, almost all vehicles for sale today comprise some sort of exhaust aftertreatment system. Examples are catalytic converters, particulate filters and Selective catalytic reduction (SCR) arrangements. A selective catalytic reduction arrangement is a means of converting nitrogen oxides, also referred to as NOx with the aid of a catalyst into diatomic nitrogen N2, and water H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of exhaust gas and is adsorbed onto a catalyst. Carbon dioxide, CO2 is a reaction product when urea is used as the reductant.
Function of these exhaust aftertreatment systems rely on the high temperature of the exhaust gases. Problems may arise upon long lasting engine braking operations since the gases leaving the engine during engine braking is cooler than exhaust gases produced in a power producing mode, which gases may cool the exhaust aftertreatment system to a temperature in which it may not function properly.
In view of above, there is a need for an improved engine braking of a four-stroke internal combustion engine.